Chromosomal recombination resulting in a gene fusion between a strong promoter and an ETS- family transcription factor ERG is the most common genetic event in human prostate cancer. The significance of ERG overexpression and the mechanisms of ERG are not well understood. This application aims at answering these questions and builds on extensive preliminary evidence generated in our laboratory. We generated transgenic mice overexpressing ERG in mouse prostate epithelium at the levels similar to the levels of ERG in human prostate cancer, and found that these animals develop age-related prostate cancer. This is exciting, because age is the most significant factor in development of human prostate cancer. The analyses of ERG transgenic animals revealed specific activation of the genes negatively controlled by the Hippo signaling pathway. Hippo pathway is known to restrict self-renewal of stem and progenitor cells and constitutive inactivation of the Hippo pathway is causally involved in cancer in liver, skin, intestine and breast epithelium. The significance of the Hippo pathway in prostate epithelium and prostate cancer is presently unknown. We confirmed ERG-dependent activation of Hippo genes in human prostate epithelial cells and found that ERG binds to Hippo transcriptional co-activators YAP1/TAZ and potentiates their transcriptional activity. Thus, the central hypothesis of this application is that the Hippo pathway plays an important role in regulation of homeostasis of normal prostate gland and activation of Hippo target genes is the primary pathway responsible for oncogenic function of ERG in prostate cancer. We propose to comprehensively investigate this hypothesis using in vitro and in vivo approaches. In aim 1, we will investigate the physical and functional connection between ERG and YAP1/TAZ proteins. In aim 2, we will determine the mechanisms and the impact of ERG on the transcriptional output of the Hippo pathway. In aim 3, we will use genetically engineered mice to reveal the significance of the Hippo pathway in normal prostate gland homeostasis and ERG- mediated prostate cancer. Overall, this study will determine the role of the Hippo pathway in normal prostate and reveal the pivotal mechanisms responsible for the development of prostate cancer.